The 2/3 subway to Grand Army Plaza is less than 1/2 mile from the start and the Q stop at 7th Avenue is not much further. Note that the 2/3 stop at Eastern Parkway - Brooklyn Museum will be closed. Please check mta.info for the most up to date information on public transportation. Allow yourself plenty of time, as trains do not run as frequently on weekends
From further away, there is a Long Island Railroad stop at Atlantic Avenue-Barclays Center that is one mile from the race start. From points north and west, take Amtrak to Penn Station where you can change to the 2/3 subway to Brooklyn. You can also take Metro North trains into Grand Central Station, then change to a Brooklyn-bound 4/5 train to Atlantic Avenue-Barclay's Center.
If you are taking an UBER/Lyft/taxi to the race start, ask to be dropped off at Grand Army Plaza.
Driving to the race start is not recommended due to scarce (and expensive) parking, road closures and traffic congestion.
A parking reservation is a MUST if you intend on driving. These two garages are within walking distance of the race start and are open 24 hours.
1 Eastern Parkway
ICON/Quik Park Underhill
105 Underhill Ave